1. Field of the Invention
The present invention relates to a shadow mask used for a cathode-ray tube or the like, and more particularly, to a shadow mask having an improved resistance to vibration or shock.
2. Description of the Related Art
An example of a shadow mask 51 having a general structure is shown in FIG. 5 as an illustrated sectional view. Referring to FIG. 5, a shadow mask 51 is mounted on a cathode-ray tube for the purpose of forming a beam spot having a circular shape on a fluorescent surface or screen of the cathode-ray tube. Such a shadow mask 51 is formed with throughholes 52a and 52b, each having a prescribed shape, arranged in a prescribed pattern. The throughholes 52a and 52b are formed through an etching treatment of a metal thin sheet. In FIG. 5, the throughhole 52a is represented by a cross-sectional shape at the center portion of the shadow mask, and the throughhole 52b, by a cross-sectional shape at the peripheral portion of the shadow mask.
Each of the throughholes 52a and 52b is composed of rear side hole portions 54a and 54b through which an electron beam enters and a front side hole portions 53a and 53b through which the electron beam is emitted. The front side hole portions 53a and 53b are formed with an area larger than the rear side hole portion 54a and 54b. The front side hole portions 53a and 53b are formed with substantially uniform opening size and opening area irrespective of the forming positions on the shadow mask. The rear side hole portions 54a and 54b are as well formed with substantially uniform opening size and opening area. In the throughhole 52b on the peripheral portion of the shadow mask, the front side hole portion 53b is shifted toward the outer periphery of the shadow mask so that the electron beam is not shielded by a portion of the taper surface of the front side hole portion 53b serving as the taper surface on the outer periphery of the shadow mask 51.
When a shadow mask of the type as described above is used in a general cathode-ray tube or in a cathode-ray tube for a non-industrial TV set having a curved display screen surface, application of a dropping impact has not posed a serious problem.
However, when using the same shadow mask in a flat cathode-ray tube having a flat display screen side and a radius of curvature on the fluorescent surface side larger than the general cathode-ray tube, or when using a shadow mask having a finer pitch of throughholes or finer sizes of individual parts for achieving a higher precision in a color cathode-ray tube, a dropping impact was confirmed to cause in some cases sagging of the center portion of the shadow mask (see the broken line in FIG. 4).
In this respect, Japanese Unexamined Patent Publication No. 5-86441 discloses improving strength of a shadow mask by using a metal material having a high Young's modulus. However, since a change in the metal material itself exerts an important effect on the congeniality with the material quality and spring property of the related members such as a frame for holding the shadow mask, the inconvenience is a considerable change in materials of the related members.